Stabilization of polyacrylonitrile



United States Patent M ABSTRACT OF THE DISCLOSURE A composition which comprises polyacrylonitrile and a stabilising agent which comprises maleamic acid or an N-substituted maleamic acid in which the N-substitutants are methyl, ethyl and phenyl groups or the amide group;

or the alkali metal salts of such acids.

The invention is concerned with the stabilisation of polyacrylonitrile :and particularly with stabilisation against discolouration on exposure to heat.

By the term polyacrylonitrile I mean both homopolymers of acrylonitrile and also copolymers containing at least 80 percent by weight of acrylonitrile units. Such copolymers may be the products of copolymerisation with acrylonitrile of such other monomers :as styrene, methyl acrylate, itaconic acid, methallyl sulphonic acid and its salts, vinyl acetate and vinyl pyridine. The invention is particularly applicable to copolymers containing 90 to 95 percent by weight of acrylonitrile units.

Thermal discolouration of polyacrylonitrile has been known for some time. The discolouration is believed to be caused by conjugation of carbon-nitrogen double bonds derived from the cyanide groups. There have been many proposals for the prevention of this discolouration. These include, for example, the addition to polymer solutions of reducing agents, which may be organic or inorganic in nature. It has been found that some of the additives employed for the prevention of thermal discolouration interfere with other properties of the polyacrylonitrile, for example the dyeability may be impaired.

I have now found that the resistance of polyacrylonitrile to thermal discolouration may be improved by the presence of maleamic acid or an N-substituted maleamic acid.

Accordingly, the invention comprises a polyacrylonitrile article containing a stabilising agent which is maleamic acid or an N-substituted maleamic acid. The N-substituents of N-substituted maleamic acids suitable as stabilising agents may be, for example, -CH -C H CO.NH CO.NHR', CO.NR'R -C H or C H R'. Examples of N-substituted maleamic acids which may be employed are Maleuric acid CI-I-CO.NHOONH2 CH-COOH.

CH-COOH and Polyacrylonitrile articles to which the invention relates may take any of the usual forms. Thus, for example, such an article may be a filament, a staple fibre, a thread, a fabric or a film. Such polyacrylonitrile articles may be used in conjunction with articles consisting of other material. Thus, for example, a yarn or fabric may be a blend of polyacrylonitrile fibres with other fibres.

The stabilising agent employed in the invention may be incorporated in the polyacrylonitrile article by known methods. The most common method of application is by addition of the stabilising agent to a spinning solution 3,417,049 Patented Dec. 17, 1968 of the polyacrylonitrile. This method may be employed Where suitable, but when the solvent is an inorganic salt solution, addition of the stabilising agent thereto can lead to contamination of the solvent if a recycling process is used.

It is preferred, therefore, that the stabilising agent should be applied to the polyacrylonitrile article after the article has been formed. Thus, one method of applying the stabilising agent consists in immersing the article in 'a solution of the agent followed by removal of excess liquor from the article and by drying. In suitable cases this process may be operated continuously. It is preferred that the impregnation of the polyacrylonitrile article with the solution of the stabilising agent should be performed before the article has been dried for the first time since its production.

The amount of the stabilising agent which is incorporated in the polyacrylonitrile may vary according to such factors as the nature of the article, the degree of stabilisation required and the particular stabilising agent used. Normally, however, it is not necessary to employ more than 5 percent by weight of the compound on the article although more than this, up to 10 percent or higher, may be used. It is preferred to use a quantity of the order of 0.2 ml percent by weight.

In order to avoid undesired changes in the properties of articles treated according to the invention, it may be desirable to apply the stabilising agent in a neutralised or partially neutralised form. This is particularly the case where the polyacrylonitrile is dyeable with basic dyes. I have found that treatment with unneutralised stabilising agent considerably affects the dye uptake but that the effect may be mitigated by use of the agent in a solution which has been partially neutralised with sodium hydroxide, or other base. It is to be understood that the invention includes polyacrylonitrile articles containing the salts of maleamic acid or N-substituted maleamic acids.

The following examples illustrate the incorporation of the stabilising agents of the invention in polyacrylonitrile articles. In the Examples, the extent of thermal discolouration of the fibres is stated in terms of the fibre yellowness which is determined on a Colourmaster differential colorimeter by measurement of the red (R), green (G), and blue (B) reflectance value. The yellowness is calculated from the equation:

Example 1 A 200 filament, 600 denier, bright polyacrylonitrile tow was passed continuously, before it had been dried after production, through a bath containing a 1 percent by weight aqueous solution of maleuric acid, which had been neutralised to pH 4.5 with sodium hydroxide. After im' mersion in the bath, the tow was padded to percent take-up and dried in an oven at 80 C.

A sample of the treated polyacrylonitrile tow and a sample of the same material which had not been so treated were exposed to a temperature of C. for one hour in an oven after which the yellownesss of the samples was determined. The following results were obtained:

Fibre yellowness The procedure of Example 1 was followed with the exception that the maleuric acid was replaced by maleamic 3 acid. The results obtained are shown in the following table:

Fibre yellowness Before heating After heating Untreated sample 0.076 0.33 Treated sample 0. 071 0. 225

Example 3 The procedure of Example 1 was followed with the exception that the pH of the solution was varied, the results being as follows:

Fibre yellowness pH Before heating After heating Example 4 The procedure of Example 1 was followed with the exception that the fibre treated had been dried before it was contacted with the solution containing the maleuric acid. The fibre yellowness increased from 0.10 to 0.245 on heating but showed an improvement compared with untreated material.

Example 5 which were stretched and dried and tested to measure their tability to thermal discolouration. The fibres were heated for one hour in an oven at 130 C. during which the fibre yellowness was found to rise from 0.075 to 0.297 compared with a final fibre yellowness of 0.35 for untreated fibre.

I claim:

1. A composition which comprises polyacrylonitrile and a stabilising agent selected from the group consisting of (a) maleamic acid;

(b) an N-substituted maleamic acid in which the N-substituent is selected from the group consisting Of CH3, 'C2H5, -CONH2, and C6H5; and

(c) alkali metal salts of (a) and (b).

2. A composition as claimed in claim 1 in which the stabilising agent is maleuric acid.

3. A composition as claimed in claim 1 in which the stabilising agent is maleanilic acid.

4. A composition as claimed in claim 1 which contains from 0.1 to 5 percent by weight of the stabilising agent.

5. A composition as claimed in claim 1 which contains from 0.2 to 1 percent by weight of stabilising agent.

References Cited UNITED STATES PATENTS 2,732,363 1/1956 Coover et al. 260-455 3,210,315 10/1965 Blackburn et al. 260-4575 3,318,983 5/1967 Hovis et al. 264l82 DONALD E. CZAJA, Primary Examiner.

V. P. HOKE, Assistant Examiner. 

